This invention relates to tubular graft structures for replacing or supplementing a patient""s natural body organ tubing. More particularly, the invention relates to tubular graft structures in which the elastic compliance of the graft varies along the length of the graft.
A patient""s weakened or diseased body organ tubing can often be repaired by replacing or supplementing the patient""s existing natural body organ tubing with an artificial graft structure. One of the goals in using artificial grafts to repair natural body organ tubing is to match the characteristics of the artificial graft to those of the natural graft as closely as possible. For example, an important property of artificial grafts used to repair blood vessels is that they be distensible like natural blood vessels. Distensible grafts are less susceptible to blood clot formation than other grafts, because distensible grafts pulsate during blood flow, which tends to hinder blood clot formation. As described in Goldsteen et al. U.S. patent application Ser. No. 08/839,080, filed Apr. 23, 1997, distensible grafts may be formed from a nitinol mesh frame covered with a silicone coating.
The natural distensibility of an artery allows energy to be stored in the walls of the artery during periods of systolic blood pressure and allows energy to be released from the walls during periods of diastolic blood pressure. Storage and subsequent release of energy by the distensible artery walls helps to sustain blood flow.
The distensibility of a given portion of natural body organ tubing or artificial graft tubing can be quantified by its compliance, which is defined as the elastic change in diameter of the tubing per unit fluid pressure inside the tubing. The compliance of an artery is determined by the amount of elastin fibers in the artery wall. The downstream or distal portions of the artery are typically less compliant than the upstream or proximal portions of the artery.
This gradient in the compliance of the artery allows the upstream portions of the artery to match the relatively high compliance of vessels in the upstream artery environment and allows the downstream portions of the artery to match the lower compliance of the peripheral blood vessel beds fed by the downstream portions of the artery. Because the compliance of each portion of the artery is matched to the compliance of the blood vessels connected to that portion of the artery, stress and possible damage to the artery walls due to abrupt transitions in compliance is reduced.
It is therefore an object of the present invention to provide a distensible artificial graft having compliance properties similar to the compliance properties of the natural body organ tubing of a patient.
It is also an object of the present invention to provide a distensible artificial graft that has a compliance gradient and is compliance matched to the body organ tubing of a patient.
These and other objects of the invention are accomplished in accordance with the principles of the present invention by providing a distensible artificial graft that may be used to replace or; supplement diseased or damaged natural body organ tubing. For example, the graft may be used to repair blocked blood vessels. Because the graft is distensible, in vascular applications the graft pulsates like natural blood vessels, which may reduce the incidence of blood clot formation.
The graft has a compliance (i.e., change in diameter of the graft per unit pressure inside the graft) that varies along the length of the graft. This compliance gradient allows the graft to create a smooth transition between portions of body organ tubing with different compliances. For example, the graft may be used to connect an upstream portion of an artery (which has a relatively high compliance) with a downstream portion of the artery (which has a relatively low compliance). By matching the magnitude of the compliance at each end of the graft with the portion of body organ tubing to which it is connected, abrupt transitions in compliance are avoided. Avoiding such abrupt transitions reduces stress and possible damage to the body organ tubing in the vicinity of the graft.
The graft may be formed from any suitable distensible tubular structure in which compliance can be varied along the length of the structure. For example, the graft may be formed from a flexible tubular mesh frame covered with an elastic coating. A suitable mesh may be formed from nitinol wire. A suitable coating is silicone.
The compliance gradient may be formed by varying the density of the mesh along the length of the graft. Higher density mesh is generally less compliant than lower density mesh. Mesh density can be controlled during graft fabrication by varying the pattern of the mesh. For example, a tighter weave or braid increases the density of the mesh. Preferably, the density of the mesh is controlled by varying the pic count of the mesh. Other techniques that may be used to control the density of the mesh include varying the size of the nitinol wire and varying the number of wire strands that are used to form the mesh.
If desired, the compliance gradient may be formed by varying the thickness of the elastic coating used to cover the frame. Portions of the graft where the coating is thick are less compliant than portions of the graft where the coating is thin. If the graft is formed primarily from a single material (e.g., a polymeric substance), the graft compliance can be controlled by varying the thickness of the material.
A compliance gradient may be created by compressing a conical frame into a cylindrical graft shape. The conical frame may be formed on a conical mandrel. If a heat sensitive memory-effect metal such as nitinol is used as the frame material, the frame may initially be formed in a cylindrical shape and subsequently stretched and heat-set in the desired conical shape. After the conical frame shape is created, the frame is radially compressed into a cylindrical shape and covered with a suitable coating such as silicone. The portions of the frame that were the largest radially before compression contribute a radial outward bias to the completed graft structure. The outward bias of such frame portions increase the compliance of the corresponding portions of the graft.
Another way in which to create the compliance gradient for the graft is to vary the properties of the materials used to form the graft. For example, coatings of different durometer or Young""s modulus may be used to cover different portions of a frame structure. If desired, the porosity of the graft may be varied to create the compliance gradient.
Distensible connector structures may be used to attach the graft to the body organ tubing. One suitable distensible connector structure is an elastic ring with radially extending barbs or hooks. When the graft is installed in the patient, the elastic ring expands to force the barbs through the graft and into the body organ tubing, thereby attaching the graft to the body organ tubing. If desired, the compliance of such connector structures can be matched to the compliance of the body organ tubing at the attachment site.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.